This invention relates to the hot and warm rolling of thin strip typically above 700° C. It has particular, but not exclusive, application to in-line rolling of thin steel strip produced by direct casting with a twin roll caster where shape correction of the strip is important.
Recent developments in twin roll strip casting have enabled steel strip to be produced to thickness of the order of less than 5 mm and typically 3 mm and less. Such strip can be further reduced in thickness by reduction in an in-line hot rolling mill as it is produced from the caster. It has been found that when hot rolling such strip to further reduce the thickness significant defects can be generated in the strip due to crimping of the strip material on entry to the work rolls of the mill. Such crimping defects may be relatively minor and appear as curved lines seen on the surface of the strip. However, particularly when rolling very thin strip, the crimped parts of the strip may become folded over prior to rolling so that parts of the strip become heavily reduced and split with resulting very severe defects.
Such crimping defects have been found to be due to variations in the strip thickness and the resulting reduction across the width of the strip. Typically, the center part of the strip may be subjected to higher percentage reduction than the strip edges, or “waves” may occur across the width of the strip. The latter manifests as “waves” along the length of the strip. The action of reduction through the rolling mill creates more backwards slip in the thicker portions of the strip relative to the thin portions of the strip. The thicker parts of the strip will therefore be subject to lengthwise compression whereas the thinner parts will be subjected to tension, and that will cause buckling. The buckles are then rolled into the strip to create downstream shape defects. In extreme cases the strip may completely fold over at the buckles and the folded over material is rolled-in to produce severe defects. Where variations in strip thickness across the strip width are localized to small parts of the strip width, the result can be localized crimping of the strip. The extent of the crimping is related to the size of the difference in thickness across the strip width and the extent of the strip width affected by the difference in reduction.
We have found that crimping during hot rolling of thin steel strip of less than about 3 mm thickness can be substantially controlled by ensuring that the strip entering the rolling mill is subjected to tension within specific limits. More specifically, we have found it is possible by applying certain tension that buckling of the strip of the kind which will initiate crimping can be avoided, while at the same time, maintaining the tension below an upper limit to avoid excessive strip creep (leading to necking or tearing) that will damage the strip.
An illustrative embodiment of the invention provides a method of shape correction by hot rolling of thin steel strip of the type most typically produced by direct casting using a twin roll caster, comprising feeding the strip through a roll bite between a pair of work rolls, applying strip squeezing forces between the work rolls to reduce the thickness of the strip, and applying tension to the strip passing to the work rolls sufficiently high to ensure no part of the strip entering the work rolls is in longitudinal compression such as to exceed the buckling stress of the strip and sufficiently low as to produce no more than 1% strip elongation through creep. The tension allowable to produce no more than 1% elongation through creep was determined as described in “Effect of Carbon Content on Plastic Flow of Plain Carbon Steel at Elevated Temperatures”, P. J. Wray, American Society for Metals and the Metallurgical Society of AIME, Vol 13 (January 1982).
While the strip may be in excess of 2.5 mm thick prior to rolling, it may also have a pre-rolling thickness of as low as about 0.5 mm or less.
The strip may be hot rolled at a temperature of at least about 700° C. The strip may also be hot rolled up to a temperature of about 1200° C. The degree of reduction of the strip thickness through the work rolls is generally about 35% or less, and is determined usually by the customer choice of thickness in the final strip.
The applied tension may be such as to limit the strip elongation through creep to no more than 0.5%.
Preferably, the variation in strip thickness reduction across the strip imposed by the work rolls is sufficiently small to prevent strip shape defects and surface creasing downstream of the work rolls of more than 200 I-units. However, in some circumstances, the variation in reduction may be such as to permit downstream shape defects and surface creasing of up to 400 I-units. The produced strip will vary in shape defects across its width; these I-unit figures are the worst case shape defects and surface creasing of the strip. The rolled strip so produced is typically additionally processed after cooling in a commercial skin pass mill to produce flatter strip with shape defects and surface creasing below 100 I-units.
I-units are a measure of the flatness of the strip produced. I-units are determined by the equation:I-units=(h/l)2×24.649where “h” is the peak to peak amplitude, and “l” is the distance between peaks (i.e. wavelength) of the shape defects in the strip. Sometimes by convention in operation 24.649 is rounded to 25 in use of this equation in determining the I-units.
The tension is typically applied to the strip by passing it through a pair of pinch rolls in advance of the work rolls, but additional pinch rolls may be also be used downstream of the work rolls to maintain tension across the work rolls. The greater the tension at the work rolls the lesser the rolling load to achieve a given reduction.